Long-term monitoring data show that hard coral cover on the Great Barrier Reef (GBR) has reduced by >70 % over the past century. Although authorities and many marine scientists were in denial for many years, it is now widely accepted that this reduction is largely attributable to the chronic state of eutrophication that exists throughout most of the GBR. Some reefs in the far northern GBR where the annual mean chlorophyll a (Chl a) is in the lower range of the proposed Eutrophication Threshold Concentration for Chl a (~0.2–0.3 mg m−3) show little or no evidence of degradation over the past century. However, the available evidence suggests that coral diseases and the crown-of-thorns starfish will proliferate in such waters and hence the mandated eutrophication Trigger values for Chl a (~0.4–0.45 mg m−3) will need to be decreased to ~0.2 mg m−3 for sustaining coral reef communities.

New study suggests coral reefs may be able to adapt to moderate climate change

Coral reefs may be able to adapt to moderate climate warming, improving their chance of surviving through the end of this century, if there are large reductions in carbon dioxide emissions, according to a study funded by NOAA and conducted by the agency’s scientists and its academic partners. Results further suggest corals have already adapted to part of the warming that has occurred. “Earlier modeling work suggested that coral reefs would be gone by the middle of this century.

Our study shows that if corals can adapt to warming that has occurred over the past 40 to 60 years, some coral reefs may persist through the end of this century,” said study lead author Cheryl Logan, Ph.D., an assistant professor in California State University Monterey Bay’s Division of Science and Environmental Policy. The scientists from the university, and from the University of British Columbia, were NOAA’s partners in the study. Warm water can contribute to a potentially fatal process known as coral “bleaching,” in which reef-building corals eject algae living inside their tissues. Corals bleach when oceans warm only 1-2°C (2-4°F) above normal summertime temperatures. Because those algae supply the coral with most of its food, prolonged bleaching and associated disease often kills corals.

The study, published online in the journal Global Change Biology, explores a range of possible coral adaptive responses to thermal stress previously identified by the scientific community. It suggests that coral reefs may be more resilient than previously thought due to past studies that did not consider effects of possible adaptation. The study projected that, through genetic adaptation, the reefs could reduce the currently projected rate of temperature-induced bleaching by 20 to 80 percent of levels expected by the year 2100, if there are large reductions in carbon dioxide emissions.

[…] In the study, researchers used global sea surface temperature output from the NOAA/GFDL Earth System Model-2 for the pre-industrial period though 2100 to project rates of coral bleaching. Because initial results showed that past temperature increases should have bleached reefs more often than has actually occurred, researchers looked into ways that corals may be able to adapt to warming and delay the bleaching process. […].

Research led by Stanford scientist Steve Palumbi reveals how some corals can quickly switch on or off certain genes in order to survive in warmer-than-average tidal waters.

By Rob Jordan

To most people, 86-degree Fahrenheit water is pleasant for bathing and swimming. To most sea creatures, however, it’s deadly. As climate change heats up ocean temperatures, the future of species such as coral, which provides sustenance and livelihoods to a billion people, is threatened. Through an innovative experiment, Stanford researchers led by biology Professor Steve Palumbi have shown that some corals can – on the fly – adjust their internal functions to tolerate hot water 50 times faster than they would adapt through evolutionary change alone. The findings, published April 24 in Science, open a new realm of possibility for understanding and conserving corals. „The temperature of coral reefs is variable, so it stands to reason that corals should have some capacity to respond to different heat levels,“ said Palumbi, director of Stanford’s Hopkins Marine Station and a senior fellow at the Stanford Woods Institute for the Environment. „Our study shows they can, and it may help them in the future as the ocean warms.“

Coral reefs are crucial sources of fisheries, aquaculture and storm protection. Overfishing and pollution, along with heat and increased acidity brought on by climate change, have wiped out half of the world’s reef-building corals during the past 20 years. Even atemporary rise in temperature of a few degrees can kill corals across miles of reef. American Samoa presents a unique case study in how corals might survive a world reshaped by climate change. Water temperatures in some shallow reefs there can reach 95 degrees Fahrenheit, enough to kill most corals. To find out how native corals survive the heat, researchers in Palumbi’s lab transplanted colonies from a warm pool to a nearby cool pool and vice versa.

The researchers found that, over time, cool-pool corals transplanted to the hot pool became more heat-tolerant. Although these corals were only about half as heat-tolerant as corals that had been living in the hot pool all along, they quickly achieved the same heat tolerance that could be expected from evolution over many generations. Corals, like people, have adaptive genes that can be turned on or off when external conditions change. The corals Palumbi’s group studied adjusted themselves by switching on or off certain genes, depending on the local temperature. These findings make clear that some corals can stave off the effects of ocean warming through a double-decker combination of adaptation based on genetic makeup and physiological adjustment to local conditions.

„These results tell us that both nature and nurture play a role in deciding how heat-tolerant a coral colony is,“ Palumbi said. „Nurture, the effect of environment, can change heat tolerance much more quickly – within the lifetime of one coral rather than over many generations.“ Palumbi cautioned that corals‘ heat-adaptive characteristics do not provide a magic bullet to combat climate change. They can’t respond to indefinite temperature increases and they could be compromised by stressors such as acidification and pollution. Still, if it holds true for most corals, this adaptive ability could provide a „cushion“ for survival and might give coral reefs a few extra decades of fighting back the harsh effects of climate change, Palumbi said.

Remote reefs can be tougher than they look: Western Australia’s Scott Reef has recovered from mass bleaching in 1998.

Isolated coral reefs can recover from catastrophic damage as effectively as those with nearby undisturbed neighbours, a long-term study by marine biologists from the Australian Institute of Marine Science (AIMS) and the ARC Centre of Excellence for Coral Reef Studies (CoECRS) has shown. Scott Reef, a remote coral system in the Indian Ocean, has largely recovered from a catastrophic mass bleaching event in 1998, according to the study published in Science today.

The study challenges conventional wisdom that suggested isolated reefs were more vulnerable to disturbance, because they were thought to depend on recolonisation from other reefs. Instead, the scientists found that the isolation of reefs allowed surviving corals to rapidly grow and propagate in the absence of human interference.

Australia’s largest oceanic reef system, Scott Reef, is relatively isolated, sitting out in the Indian Ocean some 250 km from the remote coastline of north Western Australia (WA). Prospects for the reef looked gloomy when in 1998 it suffered catastrophic mass bleaching, losing around 80% of its coral cover. The study shows that it took just 12 years to recover. Spanning 15 years, data collected and analysed by the researchers shows how after the 1998 mass bleaching the few remaining corals provided low numbers of recruits (new corals) for Scott Reef. On that basis recovery was projected to take decades, yet within 12 years the cover and diversity of corals had recovered to levels similar to those seen pre-bleaching.

“The initial projections for Scott Reef were not optimistic,” says Dr James Gilmour from AIMS, the lead author on the publication, “because, unlike reefs on the Great Barrier Reef, there were few if any reefs nearby capable of supplying new recruits to replenish the lost corals at Scott Reef. “However, the few small corals that did settle at Scott Reef had excellent rates of survival and growth, whereas on many nearshore reefs high levels of algae and sediment, and poor water quality will often suppress this recovery.

“We know from other studies that the resilience of reefs can be improved by addressing human pressures such as water quality and overfishing,” says Dr Gilmour. “So it is likely that a key factor in the rapid recovery at Scott Reef was the high water clarity and quality in this remote and offshore location.” Dr Andrew Heyward, Principal Research Scientist at AIMS, highlights another conclusion from their findings. “Previously we’ve tended to factor proximity to other reefs as a key attribute when estimating the resilience of a reef following a major disturbance, but our data suggests that given the right conditions, reefs might do much of the recovery by themselves.” This finding could have implications for the management of marine protected areas.

In their publication the team also draws attention to the important role played by climate change in the longer-term prospects for coral reefs, as Prof Morgan Pratchett of CoECRS explains. “While it is encouraging to see such clear recovery, we need to be mindful of the fact that the coral recovery at Scott Reef still took over a decade. If, as the climate change trend suggests, we start to see coral bleaching and other related disturbances occurring more frequently, then reefs may experience a ratcheting down effect, never fully recovering before they suffer another major disturbance. “By preventing illegal fishing and enhancing water quality on coral reefs in all regions we will give these reefs a greater capacity to recover from major disturbances.”

The highly detailed, long-term data set makes Scott Reef the best studied reef in Australia’s Indian Ocean territory. The study provides valuable new perspectives on ecosystem function and resilience of coral reefs situated in the northwest Australia, and in other contexts such as the Great Barrier Reef, and illustrates the importance of AIMS’ research collaborations with its industry partners.

Recurrent disturbances, recovery trajectories, and resilience of coral assemblages on a South Central Pacific reefCoral reefs are increasingly threatened by various disturbances, and a critical challenge is to determine their ability for resistance and resilience. Coral assemblages in Moorea, French Polynesia, have been impacted by multiple disturbances (one cyclone and four bleaching events between 1991 and 2006). The 1991 disturbances caused large declines in coral cover (~51% to ~22%), and subsequent colonization by turf algae (~16% to ~49%), but this phase-shift from coral to algal dominance has not persisted. Instead, the composition of the coral community changed following the disturbances, notably favoring an increased cover of Porites, reduced cover of Montipora and Pocillopora, and a full return of Acropora; in this form, the reef returned to pre-disturbance coral cover within a decade. Thus, this coral assemblage is characterized by resilience in terms of coral cover, but plasticity in terms of community composition.

Resistance to thermal stress in corals without changes in symbiont compositionDiscovering how corals can adjust their thermal sensitivity in the context of global climate change is important in understanding the long-term persistence of coral reefs. In this study, we showed that short-term preconditioning to higher temperatures, 3°C below the experimentally determined bleaching threshold, for a period of 10 days provides thermal tolerance for the symbiosis stability between the scleractinian coral, Acropora millepora and Symbiodinium. Based on genotypic analysis, our results indicate that the acclimatization of this coral species to thermal stress does not come down to simple changes in Symbiodinium and/or the bacterial communities that associate with reef-building corals. This suggests that the physiological plasticity of the host and/or symbiotic components appears to play an important role in responding to ocean warming. The further study of host and symbiont physiology, both of Symbiodinium and prokaryotes, is of paramount importance in the context of global climate change, as mechanisms for rapid holobiont acclimatization will become increasingly important to the long-standing persistence of coral reefs.

Historical Temperature Variability Affects Coral Response to Heat StressCoral bleaching is the breakdown of symbiosis between coral animal hosts and their dinoflagellate algae symbionts in response to environmental stress. On large spatial scales, heat stress is the most common factor causing bleaching, which is predicted to increase in frequency and severity as the climate warms. There is evidence that the temperature threshold at which bleaching occurs varies with local environmental conditions and background climate conditions. We investigated the influence of past temperature variability on coral susceptibility to bleaching, using the natural gradient in peak temperature variability in the Gilbert Islands, Republic of Kiribati. The spatial pattern in skeletal growth rates and partial mortality scars found in massive Porites sp. across the central and northern islands suggests that corals subject to larger year-to-year fluctuations in maximum ocean temperature were more resistant to a 2004 warm-water event. In addition, a subsequent 2009 warm event had a disproportionately larger impact on those corals from the island with lower historical heat stress, as indicated by lower concentrations of triacylglycerol, a lipid utilized for energy, as well as thinner tissue in those corals. This study indicates that coral reefs in locations with more frequent warm events may be more resilient to future warming, and protection measures may be more effective in these regions.